The present invention relates to skeletal tissue stimulators, more particularly bone growth stimulators and still more particularly to low voltage oscillator circuits for use in such stimulators.
It is known that, in certain circumstances, the application of an electrical current to skeletal tissue, especially bone tissue, may promote growth of that skeletal tissue. This is particularly useful in situations of non-union or delayed union of fractures of bones.
In one type of a typical skeletal tissue stimulator, a pair of electrodes are invasively inserted near the fracture site. These electrodes are then connected to an electrical circuit which passes electrical current between the electrodes and, hence, to the bone tissue. The electrical circuit determines the amount of and the characteristics of the electrical current which is passed to the electrodes and which is then utilized to stimulate the skeletal tissue.
A variety of electrical current wave forms have been used for skeletal tissue, especially bone, stimulation. In the past, electrical currents involving direct current wave forms and alternating current wave forms have been used. Alternating current wave forms with differing amplitudes, frequencies, duty cycles and average DC levels have been utilized. One example of an electrical current which has been found to be useful in skeletal tissue stimulators is an electrical pulsed current wave form of approximately 20 microamperes with approximately 50% duty cycle. In this electrical current wave form, an approximate DC current is applied for approximately one half of the time and a low electrical current is applied for remaining approximately one half time. Such an electrical current wave form then resembles an approximate square wave with a DC shift of approximately one half of the peak-to-peak value.
In certain skeletal tissue, e.g. bone, stimulators, it is desired to implant the entire stimulation unit in order to avoid a percutaneous connection between the electrodes and the stimulation unit. In this situation, several countervailing practical constraints tend to limit the usefulness of the stimulation device.
First, since the device is implanted or otherwise located near the site of stimulation, there is a need to have a compact unit. The compactness of the stimulation device necessarily limits its size and, hence, the capacity of its energy source, i.e. battery.
Second, since the effectiveness of the stimulation is, to a certain extent, the result of the magnitude of the electrical current induced into the skeletal tissue, the circuit must be capable of supplying and maintaining an electrical current at that level. This requirement militates toward a larger energy source, i.e. battery.
Third, since the device is invasive, i.e. implanted near the skeletal tissue to be stimulated, it is desirable to extend the lifetime of the energy source, i.e. battery, in order to achieve a maximum amount of stimulation with a minimum amount of use of invasive procedures.